The mobile or portable computer segment of the personal computer marketplace has grown rapidly over the recent past and has become a mainstream product area rather than a niche. This growth has been fueled by the design of products which allow the user to run the same applications with substantially the same performance as the desktop versions of personal computers.
Portable computers are equipped with a built-in Liquid Crystal Display (LCD) and typically a connector for use with an external CRT monitor. When operating in a mobile environment on battery power, the user must rely solely on the LCD display. The viewing quality of this display is dependent on the LCD technology employed and the display controller in the portable computer which drives the display. Although monochrome LCD displays remain in use at the low end of the product line, most portable computers today incorporate a color LCD.
The color LCD display panels in current products generally fall into two basic types--active matrix and passive matrix. In the active matrix display panel, a trio of thin film transistors (TFT) is paired with liquid crystal elements to activate each pixel. A matrix of color filters representing Red, Green and Blue light is arranged to provide the resultant color display. Active matrix flat panel displays exhibit superior display quality over passive matrix technology, but pay a penalty in power consumption, resulting in shorter battery life in portable computers, and are significantly more costly due to the relatively low yield achieved in production.
The passive matrix display panel is composed of an orthogonal matrix of conductive stripes or electrodes between sheets of glass with the intersections of the stripes defining a matrix of displayable locations corresponding to pixels. The matrix is scanned by applying voltage to the rows and columns in sequence, a complete scan of the matrix being termed a frame. As each pixel is scanned, it becomes visible by means of polarizing light transmitted through the panel from a source known as the backlight. For a color display, white light is passed through layers of filters controlled by liquid crystal material. The color of a scanned pixel is determined by the polarization of liquid crystal material in the filters, which is in turn controlled by color code inputs to the panel. The passive matrix panel is considerably less costly with lower power consumption than the active matrix panel, albeit with inferior display quality. The passive matrix LCD continues to be used in a large portion of the portable computer market.
Because the viewing quality of the passive matrix display panel is greatly dependent on refresh rates, recent advances in the technology have produced a version of the passive matrix panel which divides the display into an upper and lower panel. Data is provided to each panel simultaneously, in effect doubling the refresh or frame rate and achieving a corresponding improvement in the viewing quality of the display without a significant increase in cost. This type of panel is termed a Dual Scan panel since the two panels are scanned simultaneously.
The Dual Scan panel imposes a unique burden on the LCD controller used to generate the display in the portable computer. In prior art controllers, the data displayed on the LCD was directly derived by capturing the sequential data stream targeted for the CRT and converting the data to LCD format. As a consequence, the refresh frame rate for the panel was identical to that set for the CRT. As the Dual Scan panel requires the simultaneous presentation of data from two disjoint areas of display memory, the straightforward sequential data capture method is not workable.
Several solutions to this problem have been implemented. Some prior art controllers have implemented a frame buffer technique, wherein the data for the panel is captured in a separate buffer. The LCD panel controller logic then accesses the frame buffer to acquire the data for both upper and lower panels. This results in the panel refresh rate matching that of the CRT. The problem is that the passive matrix generally requires a refresh rate which is significantly higher than that tolerated by the CRT for optimum viewing quality. Another problem is that the prior art controllers processed the color data through a dithering engine prior to storage in the frame buffer. The dithering engine must be tightly coupled with the display refresh in order to achieve the maximum benefit of perceived color expansion. This would not be the case when the frame buffer stores dithered data.
An improvement in the prior art was the use of frame acceleration. In this architecture, the panel is refreshed from two sources--the CRT data stream and the frame buffer. During the first half of the display frame, the upper half of the panel is refreshed from the CRT data stream while the lower half of the panel is refreshed from the frame buffer. At the point of crossover where the CRT display is at the scan line following the last upper panel line, the refresh sources are reversed. This results in the panel being refreshed at twice the CRT rate, resulting in an improved display viewing quality. One problem with this approach is that the dithered data becomes displaced by one frame in each half of the panel, resulting in limitations and disadvantageous use of the dithering engine. Another problem is a compromise in the optimum refresh rate. Either the CRT or the LCD panel viewing quality may have to be sub-optimal to compensate for the other device.
A variation on the frame acceleration approach used in the prior art was the use of a half frame buffer. One of the panels is refreshed by data from the CRT display stream, while the other panel is refreshed from the half-frame buffer separately. The display memory and the half-frame buffer are accessed synchronously to maintain coherence of the display. A problem remaining with this solution is that, particularly when the CRT and LCD panel displays are concurrently active, the refresh frame rate available to the LCD panel is governed by the CRT parameters and cannot be optimized for panel requirements. Another problem is that the power consumption of the video subsystem is proportional to the clock rates at which the logic is driven. The rates required by the CRT force the power consumption of the LCD sub-system to be higher than necessary. Another problem arises because the half-frame buffer is linked to a specific area of display memory. As most portable computers comply with the Video Graphics Adapter (VGA) standard, market requirements dictate that strict compatibility must be maintained. An operation involving the Line Compare Register operation conflicts with the half frame buffer approach. In this operation, a scan line count can be set in the controller which, when the preset number of scan lines have been displayed, resets the display memory pointer to the start of memory. This allows the scrolling of part of the display, while the remainder is static. If, as an example, the half frame buffer is a copy of the last half of display memory, only that portion of memory can be displayed on the lower panel. If the line compare operation is active, the lower panel might be required to display a portion of the first half of display memory. The controller would then not allow strict compatibility with VGA standards.
An alternate solution to the Dual Scan problem is given in U.S. Pat. No. 5,387,923. The patent discloses a controller using address translation logic to interleave data written by the host computer sequentially in display memory, thereby alternating upper and lower panel data as memory is sequentially accessed. This eliminates the need for an additional buffer but again does not solve the problem of optimizing LCD refresh rates and VGA compatibility problems, such as Line Compare, are not addressed.
Accordingly, the present invention provides a display controller which provides refresh rates which may be optimized for viewing quality on a plurality of display mechanisms without detrimental interaction. The present invention also maintains compatibility with graphics standards while providing the improved display quality.
In view of the shortcomings of the prior art, there exists a need for an improved mechanism for optimally driving both a CRT and an LCD simultaneously.